Magnetic field sensors that sense motion of a target are known. The target can be a ferromagnetic target or a magnetic target (e.g. a ring magnet). Often, the sensed target motion is rotation. Such sensors detect features of the rotating target's profile, for example, teeth and valleys of a ferrous gear or north and south poles of a magnetic target such as a ring magnet.
The magnetic field associated with the target profile is sensed by a magnetic field sensing element, such as Hall element or magnetoresistive (MR) element. As the target passes the sensing element, the magnetics field experienced by the sensing element varies in relation to the target profile. The sensing element provides a signal proportional to the sensed magnetic field and the sensor processes the magnetic field signal to generate an output, for example, a signal that changes state each time the magnetic field signal crosses a threshold. Such an output can be used to provide rotational speed information. A second sensing element can be employed to generate an output for rotational direction detection as well.
Some sensors, referred to as differential sensors, contain two sensing elements configured in a differential arrangement. In differential magnetic field sensors, the difference between the signals provided by the two sensing elements is used to generate a differential magnetic field signal indicative of the target's features. As the differential magnetic field sensor only responds to changes in magnetic field strength, it is relatively immune to interference. Differential sensors containing three or more sensing elements can be used to provide rotational speed and direction information.
Unlike the non-differential sensing type magnetic field sensor, the differential magnetic field sensor is orientation dependent with respect to the target. Thus, when a differential magnetic field sensor is used to measure the speed (or direction) of a rotating target, the pair of sensing elements has to be centered over the target's profile for optimum performance. Misalignment of the sensing elements relative to the target profile results in a reduction of the peak-to-peak signal. Consequently, applications that cannot control the sensor-to-target alignment usually employ a non-differential sensing type sensor.